Influence of the Metal (Al, Cr, and Co) and the Substituents of the Porphyrin in Controlling the Reactions Involved in the Copolymerization of Propylene Oxide and Carbon Dioxide by Porphyrin Metal(III) Complexes. 2. Chromium Chemistry

The reactivities of chromium­(III) complexes LCrX, where L = 5,10,15,20-tetraphenylporphyrin (TPP), 5,10,15,20-tetrakis­(pentafluorophenyl)­porphyrin (TFPP), and 2,3,7,8,12,13,17,18-octaethylporphyrin (OEP) and X = Cl or OH, have been studied with respect to their ability to homopolymerize propylene oxide (PO) and copolymerize PO and CO₂ to yield polypropylene oxide (PPO) and polypropylene carbonate (PPC) or propylene carbonate (PC), respectively, with and without the presence of a cocatalyst, namely, 4-dimethylaminopyridine (DMAP) or PPN⁺Cl^– (bis­(triphenylphosphine)­iminium chloride). The homopolymerization is notably faster (TOF ≈ 2000 h^–1 at room temperature) than copolymerization, which commonly leads to ether-rich polymers. Studies of kinetics reveal that for TPPCr­(OH) with DMAP (1 equiv) the propagation reaction rate is first order in [Cr] with excess PO. With PPN⁺Cl^– as a cocatalyst the reaction order in [Cr] and [Cl^–] is complicated by the presence of two growing chains, and the presence of excess [Cl^–] facilitates the formation of PC by two different backbiting mechanisms. The fixation of CO₂ is promoted by [Cl^–] but is not greatly influenced by CO₂ pressure (1–50 bar). The reactions and polymers have been monitored by UV–visible spectroscopy, react-IR, GPC, ESI, and MALDI TOF, and NMR (¹H, ¹³C­{¹H}) spectroscopy. Notable differences are seen in these reactions when compared with earlier studies by Darensbourg et al. with salen chromium­(III) systems and related aluminum­(III) porphyrins

Coupling of Propylene Oxide and Lactide at a Porphyrin Chromium(III) Center

5,10,15,20-Tetraphenylporphyrin chromium chloride (TPPCrCl) with added [Ph₃PNPPh₃]⁺Cl^– (PPN⁺Cl^–) selectively polymerizes lactide (l and <i>rac</i>) dissolved in neat propylene oxide (PO) to yield polylactide (PLA) terminated by the −OCHMeCH₂Cl group. At 0 °C and below, <i>rac</i>-LA yields polymers highly enriched in isotactic tetrads (<i>iii</i>). At 25 °C, some stereoselectivity is lost as transesterification becomes significant, and at 60 °C and above, enchainment of PO leads to the formation of 3,6-dimethyl-1,4-dioxan-2-one by a backbiting mechanism. At 0 °C, after the enchainment of l-(<i>S</i>,<i>S</i>)-LA in neat (<i>R</i>)-(+)-PO, the formation of (3<i>S</i>,6<i>R</i>)-3,6-dimethyl-1,4-dioxan-2-one occurs, while at higher temperatures the ratio of (3<i>S</i>,6<i>R</i>)-3,6-dimethyl-1,4-dioxan-2-one to (3<i>R</i>,6<i>R</i>)-3,6-dimethyl-1,4-dioxan-2-one falls to 3:2

Coupling of Propylene Oxide and Lactide at a Porphyrin Chromium(III) Center

5,10,15,20-Tetraphenylporphyrin chromium chloride (TPPCrCl) with added [Ph₃PNPPh₃]⁺Cl^– (PPN⁺Cl^–) selectively polymerizes lactide (l and <i>rac</i>) dissolved in neat propylene oxide (PO) to yield polylactide (PLA) terminated by the −OCHMeCH₂Cl group. At 0 °C and below, <i>rac</i>-LA yields polymers highly enriched in isotactic tetrads (<i>iii</i>). At 25 °C, some stereoselectivity is lost as transesterification becomes significant, and at 60 °C and above, enchainment of PO leads to the formation of 3,6-dimethyl-1,4-dioxan-2-one by a backbiting mechanism. At 0 °C, after the enchainment of l-(<i>S</i>,<i>S</i>)-LA in neat (<i>R</i>)-(+)-PO, the formation of (3<i>S</i>,6<i>R</i>)-3,6-dimethyl-1,4-dioxan-2-one occurs, while at higher temperatures the ratio of (3<i>S</i>,6<i>R</i>)-3,6-dimethyl-1,4-dioxan-2-one to (3<i>R</i>,6<i>R</i>)-3,6-dimethyl-1,4-dioxan-2-one falls to 3:2

Influence of the Metal (Al, Cr, and Co) and Substituents of the Porphyrin in Controlling Reactions Involved in Copolymerization of Propylene Oxide and Carbon Dioxide by Porphyrin Metal(III) Complexes. 3. Cobalt Chemistry

A series of cobalt­(III) complexes LCoX, where L = 5,10,15,20-tetraphenylporphyrin (TPP), 5,10,15,20-tetrakis­(pentafluorophenyl)­porphyrin (TFPP), and 2,3,7,8,12,13,17,18-octaethylporphyirn (OEP) and X = Cl or acetate, has been investigated for homopolymerization of propylene oxide (PO) and copolymerization of PO and CO₂ to yield polypropylene oxide (PPO) and polypropylene carbonate (PPC) or propylene carbonate (PC), respectively. These reactions were carried out both with and without the presence of a cocatalyst, namely, 4-dimethylaminopyridine (DMAP) or PPN⁺Cl^– (bis­(triphenylphosphine)­iminium chloride). The PO/CO₂ copolymerization process is notably faster than PO homopolymerization. With ionic PPN⁺Cl^– cocatalyst the TPPCoOAc catalyst system grows two chains per Co center and the presence of excess [Cl^–] facilitates formation of PC by two different backbiting mechanisms during copolymerization. Formation of PPC is dependent on both [Cl^–] and the CO₂ pressure employed (1–50 bar). TPPCoCl and PO react to form TPPCo­(II) and ClCH₂CH­(Me)­OH, while with DMAP, TPPCoCl yields TPPCo­(DMAP)₂⁺Cl^–. The reactions and their polymers and other products have been monitored by various methods including react-IR, FT-IR, GPC, ESI, MALDI TOF, EXAFS, and NMR (¹H, ¹³C­{¹H}) spectroscopy. Notable differences are seen in these reactions with previous studies of (porphyrin)­M­(III) complexes (M = Al, Cr) and of the (salen)­M­(III) complexes where M = Cr, Co